1. Field of the Invention
The present invention is directed to a thin camera. More particularly, the present invention is directed to a thin camera using color filters, a thin camera having an increased field of view and/or a thin camera that can be made on a wafer level, and associated methods.
2. Description of Related Art
A conventional camera 10, shown in FIG. 1A includes a lens 12 with a focal length f and a detector array 14, having a plurality of pixels. To realize color imaging, an array of filters 15 having some pattern, typically with more green filters than red or blue filters, is provided. An example of the filter array 15 is shown in FIG. 1B. Each filter in the filter array 15 corresponds to a detector, or pixel, in the detector array 14. However, this camera is relatively thick.
One approach to realizing a thin camera with sufficient resolution involves scaling an imaging system of a conventional camera, e.g., an f/1 camera having a lens with a focal length f. Assume the focal plane has nx pixels or sensors of a size px in the x-direction and ny pixels of size py in the y-direction. This camera would then have the potential to generate an image with a resolution of nx×ny, if the imaging system had a high enough resolution. The sampling rate of the sensor plane is then equal to 1/px in the x-direction and 1/py in the y-direction. One way to measure the resolution of the imaging system is the modulation transfer function (MTF). The MTF is a measure of the contrast of spatial frequencies imaged by the optical imaging system. MTF is measured in terms of contrast as a function of spatial frequencies in lp/mm. For a sensor with pixels of dimensions of px×py, an MTF of approximately 20%-50% at typical spatial frequencies of approximately 1/(4px) and 1/(4py) in each dimension may be needed in order to obtain good quality images at the resolution of the image sensor (nx×ny).
If px and py could be reduced by a desired scaling factor, thus keeping nx and ny the same, as noted above, then f could be reduced, while maintaining the resolution. The problem is that as px and py are scaled down, higher MTF is required by the optical imaging system. It becomes harder and harder to make optical imaging systems capable of delivering the required MTF as the pixel size is scaled down. In addition, as the pixel size is reduced, other issues become more predominant including color crosstalk, electrical crosstalk, and reduced fill factor.
Another solution uses compound eye image capturing, the size of the lens being in accordance with a desired thinness of the camera. Each lens in the compound eye corresponds to a plurality of pixels, with the compound eye being selected so that spacing of the lenses is not an integral multiple of pixel spacing. Thus, each lens looks at different shifted images. The lenses used in the compound eye image capturing system generally have low resolution, for example a point spread function (PSF) much larger than the area of each pixel. A resolution greater than that of an individual sub-camera may be achieved by combining the images from multiple sub-cameras. For this solution, a color filter array 15′ shown in FIG. 1C, has a color filter for each lens. Multiple lenses may be used for each color and the images for each color combined. However, the use of compound eye image capturing is computationally intensive and it is difficult to achieve a resolution equal or close to that of the number of pixels in the sensor array for the entire composite image.
One reason for the low resolution achieved with the compound eye image capturing approach, is that a large number of lenses are used, placing practical limits on the resolution for each sub-camera. In addition, the performance of each sub-camera is also typically low due to the approach taken. That is, each sub-camera in a compound eye image capturing system typically contains one lens which is located approximately one focal length away from the sensor plane. A signal separator is often used between the lens and the focal plane to reduce crosstalk. Due to the shortness of the focal length of these systems and the thickness of typical lens substrates, there is no room in such systems to place multiple compound element lens systems in the space between the entrance pupil of the lens system and the signal separator.